Hidden in the sunken city of Aahz. Protected by Teuer-Kan, (formerly known as Ransitar). It is owned by Galap-Dreidel, who has become a demi-lich. Teuer-Kan has become a lich. Working for him are a Kraken and a pack of were-sharks.
Galap-Dreidel is spacing out in his old age, not on top of the Soul Gem much anymore, leaving that to Teuer-Kan. Teuer-Kan has gone crazy, the result of spending a couple aeons alone in astral space after refusing to leave with the githyanki. He had spent the time searching for the Ghost Tower and the Soul Gem, but he did not realize the flip-flop the universe had gone into, and the planet was unfamiliar to him when he returned. Eventually he located the planet and determined that the gem was still there. Teuer-Kan knew of the gem, because Galap-Dreidel had been a famous mage in the time of Ransitar, and they had met each other. Even then, Galap-Dreidel was incredibly high in power, and impressed Ransitar with his prophecies of the future for Set. Galap-Dreidel was at work preparing for the Invoked Devastation, which would have been an effort to weaken the imprisonment over Set and release him earlier. Galap-Dreidel went northwest to do more research, for the time of release was still about twenty years hence. Ransitar did not realize that spiritwracking the Planetar Billy-Bob would throw a monkey wrench into the works. When the Planetar was imprisoned, the four elements broke loose to do as they pleased. At the same time, the Solar got real pissed and quickly rounded up some allies for a surprise assault on Set. (score one for Chaos). When the elements moved, and the universe flipped with such resultant confusion, the surviving populace assumed that is had been the prophesied Invoked Devastation and began all kinds of false legends. Everyone who was preparing in Aahz for the day they would evacuate the planet was caught unawares when the wall of water rushed towards them out of the desert. Ransitar, after he finally located the Gem, decided that subtlety was best, since there was no longer need for hurry. He posed as a lesser-power wizard named Teuer-Kan, won the confidence of a lonely old mage, and used him as a cover as he retrieved the Soul Gem. Teuer-Kan waited, using the adventurers to gather his vital lich spell components. Then he became a lich.
Teuer-Kan fled south with the Gem, but an ancient alarm had quietly sounded. Teuer-Kan was soon confronted by the nearly disintegrated but highly active remains of Galap-Dreidel. He accepted Teuer-Kan as the gem-protector, and told him to keep filling it, to create the army of Set to support him at the Hour of his release. Galap-Dreidel told Teuer-Kan how to release the souls. But he also warned that he would know if preparations are being made early, and Galap-Dreidel still carries one vital piece of the ceremony locked in his head (literally). Teuer-Kan, already 3/4 mad from thousands of years of fruitless searching and the accompanying loneliness, and now being deprived of possession of the Gem after all that, and being saddled with the responsibilities of both trapper and guard... Well, Teuer-Kan went bonkers. He took his copy of Karnak's shade book, and became a shade, then hid the Gem in the sunken city of Aahz. With aid from Galap-Dreidel, Teuer-Kan enlisted the Kraken. (he got the were-sharks himself). The deal: the Kraken, by wind and weather, draws ships to the area near the soul gem, with its new defense system (a properly controlled Vortex, courtesy of Galap-Dreidel). It fires a brief burst, enough to suck everything on the surface down and close, but never quite too close, just nearly to the apex of the vortex. Then when the vortex cuts off, the Gem kicks in, blasting around itself until no more prey are in the area. The vortex comes from an irregularly shaped piece of green-glowing stone. The inner sea has become a terror-filled place. The Kraken has never been defeated (driven off, maybe) and none have ever been able to pursue it to its lair.
Teuer-Kan had been killed by Galap-Dreidel when they first met. But Teuer-Kan's soul went to its gem, which was on his person, and Galap-Dreidel appreciated his usefulness. Teuer-Kan could be killed, and his soul would live on in a new body. But Teuer-Kan, quite mad, used irrational logic to convince himself that as a shade-lich, he would be in better shape to survive if anything wrong happened again (with Set). He has twisted his image of Karnak into a fool who blundered in and blew apart a beautiful plan out of sheer ignorance of the larger issues, and Teuer-Kan blames Karnak for Teuer-Kan's present position, instead of living in an astral version of Aahz until the second coming of Set. Teuer-Kan distrusted Karnak's book, and figured that becoming a shade would not mean a total loss of his soul. He was wrong, and knows it, and knows that even a wish won't get it back, and knows that Galap-Dreidel is pissed and says it serves him right, and it doesn't help Teuer-Kan's sanity knowing that he is not immune to permanent death, which was the best point about becoming a lich. Teuer-Kan thought he recognized Karnak at their first meeting, but it was overshadowed by the retrieval of the Soul Gem. He realized that Karnak was unknowing of the situation, and used him to get a vital spell component. After he got it, in his excitement he offed the mage too soon and had to destroy the evidence and skip town, losing track of Karnak (momentarily, he thought). Then Galap-Dreidel jumped him, and everything else happened. He's also pissed about letting K go when he had the chance, because he could have prevented the (Un)-Invoked Devastation.
The Gem has the power to absorb a large number of souls. Galap-Dreidel and Teuer-Kan know how to summon loyal troops from the Gem. The Soul Gem is considered an Artifact.
Galap-Dreidel, in the future, knows that a god of Chaos will rampage the world shortly before Set's release. It will be one of the Cthulhu gods that blundered onto the scene (what luck!). Actually, its appearance was orchestrated by Galap-Dreidel, who had made a deal with a time elemental to take him to that time when the gem was finally filled. A scroll to summon that elemental is inside Galap-Dreidel's skull. The spell activates upon sight. The elemental will not speak to anyone, and will assume that the summoner knows what is going on. It will not complete the bargain without the rest of the payment. If it does not get the payment, it will attack and throw around through time.
Galap-Dreidel also has diabolic aid, for he kept the bargain with Mephistopheles intact and embellished on it. The deal is, if Galap-Dreidel gets the support he requests, Mephistopheles will have his best chance ever for overthrowing Asmodeus when Set arrives. After all, the bargain was originated by Set himself. Yeah, that's it. Set set the setting...
The Sunken City of Aahz will be unrecognizable to Karnak. The Soul Gem is hidden in a large cone, which is encrusted and slimed to seem like a large broken rock.
Galap-Dreidel used his wish to enable the vortex and soul-sucker to operate through the dome roof. The roof is impervious to any magic, and radiates a permanent non-detection. Nothing can teleport into the area either. Adventurers may know where the shots come from, but they can't get inside that way.
Inside the secret entrance, the place is watertight (mostly). Some areas have had air pumped in to keep the water out. Dreary, slimy, dripping hallways and rooms. Many empty, or with slimes, oozes and holds and fungi. Damp, subterranean etc.
Teuer-Kan spends his time wandering through the shadows, setting traps and other magical incidents. There are creatures from the Plane of Shadow also.
Teuer-Kan must be passed before the Soul Gem area is reached. The room is full of skeletons, bones and skulls. Also the Demi-Lich. Some fog-shrouded areas.
Galap-Dreidel and Ransitar have prepared the future to receive them and the Soul Gem. Working with other related organizations that have also awaited this day, namely the forces of Hastezini, they have assumed control of the southeast part of the map. The Chaos God will be released at the same time as the Ravers reappear. The major city of Osiris will be wiped out by the subsequent tidal wave. The Chaos God will slay tens of thousands, and to raise an army loyal to Set requires the Soul Gem.
Ransitar had created a flying chariot, to oversee the fleet and land movements. It can also sail normally, and be pulled as a wagon.